A known method of the above type is employed, for example, for the purpose of measuring signals of the central nerve system. Such a measuring process is carried out either by means of a number of electrodes which are arranged on the skull of the patient in a pattern conforming to internationally established standards (electroencephalography -- EEG), or by using a number of electrodes which are applied to the exposed cerebral cortex, or to the cerebral meninger (electrocortigraphy -- ECOG). The electrical activity of the nerve cells and the surrounding medium is picked up under the electrodes in the form of respective potential variations. The electrodes in both methods, are associated with after-connected amplifiers and recording or registering devices.
The measuring methods may be divided into bipolar and unipolar measuring processes. A bipolar measuring method comprises feeding the potential differences to the amplifier inputs which are engaged in pairs between the electrodes. In the unipolar measuring method, the potential differences are picked up between a number of electrodes and a reference point which in each case is common for said electrodes. Said reference point may be a physical electrode or, for example, the center point of a resistor network which is connected with the same resistance value to all electrodes, or optionally with the exclusion of those electrodes whose signals, on the basis of past experience, are known to adversely affect the results of the measurement because they are caused, for example, by the activity of muscles. The shortcomings of said two measuring methods are that each measured potential is the difference between two electrode potentials, which means that a selective pickup of each local change in electrode potential does not take place. Accordingly, it is not possible to exactly locate the cerebral bioelectric activity.
Furthermore, both these measuring methods are impaired by an instability factor caused by the fact that the electrode potentials are composed of the electrical activity both of the cerebral tissue disposed directly beneath the electrode and the electrical activity of an adjacent ring which is passed laterally on the skull.
The above-mentioned known measuring methods are described in greater detail later with reference to FIGS. 1 to 3.